The Taco Differential Bypass Valve provides an accurate and dependable means to control excess flow in zone valve systems that can be created when there is a reduction in the demand for heat. This reduced heat demand typically occurs as zone thermostats are satisfied and their corresponding zone valves close, causing the system pump to attempt to force more water through the remaining zones. Differential Bypass Valves should also be used to prevent dead heading of the circulator in systems where parallel piped heat emitters are controlled by thermostatic radiator valves.